Genetic analysis of a 9 kDa phycocyanin-associated linker polypeptide

The gene encoding LR9, a 9 kDa phycocyanin-associated linker polypeptide, was cloned from the cyanobacterium Synechococcus sp. PCC 7002 (Agmenellum quadruplicatum PR-6). This gene, termed cpcD, was located immediately 3′ to cpcC, a gene which encodes another phycocyanin-associated linker, LR33. Mutation of cpcD by insertion led to the loss of LR9 as the only detectable change in phycobilisome composition. Cells and isolated phycobilisomes from the cpcD^- strain did not detectably differ from the wild-type in absorption or steady-state fluorescence emission. Purified phycobilisomes from the wild-type and cpcD^- strains were compared by electron microscopy. The number of phycocyanin discs in the rod substructures of the mutant was more variable than in the wild-type. Hence, one function of LR9 may be to minimize the heterogeneity of rod length, possibly by binding to the core-distal face of phycocyanin-LR33 complexes to prevent the tandem joining of such units. A mutant in which cpcD and cpcC-cpcD intergenic sequences are deleted shows a partial loss of LR33. Inverted repeats in this intergenic region may be required for optimal stability of the cpcC transcript.